hi mike,
I have circuit, which detects the extra current that flows in the 12v line, when the motor is not rotating.
Its logic is like, 0.05 ohms resistor is kept in series with the 12v line and during the normal running conditions the drop accross the resistor would be 0.326v.
Whenever the motor is still, the current would increase, thereby increasing the voltage drop accross the resistor. The increase in drop can be identified by a comparator. The comparator output drives the LED.
Whenever the motor is still the LED glows. I have attached the circuit schematic.
Note: The circuit is not tested. This is just a rough design.